Electron Spin Resonance Spectroscopy With Femtoliter Detection Volume

We report electron spin resonance measurements of donors in silicon at millikelvin temperatures using a superconducting LC planar micro-resonator and a Josephson parametric amplifier. The resonator includes a nanowire inductor, defining a femtoliter detection volume. Due to strain in the substrate, the donor resonance lines are heavily broadened. Single-spin to photon coupling strengths up to similar to 3 kHz are observed. The single shot sensitivity is 120 +/- 24 spins/Hahn echo, corresponding to similar to 12 +/- 3 spins /root Hz for repeated acquisition.